1. Field of the Invention
The present invention relates to a pressure contact type semiconductor device used for a power converter.
2. Background of the Invention
In the field of large-capacity power electronics devices, a snubberless GCT (Gate-Commutated Turn-off) thyristor with the maximum cut-off current of 4000 A and a turn-off storage time of 3 .mu.s or less has been realized to be an alternative to a conventional GTO (Gate Turn-Off) thyristor. The operating principles and the structure of the GCT thyristor is disclosed, for example, in European Patent Publication No. EPO785627A2, Japanese Patent Laid-Open No. 8-330572A, or Mitsubishi Electric Technical Report Vol. 71, No. 12, pp. 61-66. The features of the GCT thyristor are summarized as follows: In the GCT thyristor, a gate terminal which is in contact with a ring gate electrode and extends to the outside of an insulation cylinder is changed from lead-shaped to ring-shaped; and a connection between the GCT thyristor and a gate drive circuit is improved from a lead wire structure to a laminated circuit board structure, as compared with the conventional GTO thyristor. Thus, an inductance of the gate terminal and a metal gate contact is reduced to about one hundredths of an inductance of the GTO thyristor. This makes possible an isotropic supply of reverse gate current to be passed at the time of turn-off, from all the circumferential surfaces of a gate electrode, and also a reduction in the turn-off storage time. Further, in a wafer structure of the GCT thyristor, several thousands of segments are concentrically located in parallel with each other in a several-stage configuration, and a gate electrode region forming an interface with the gate electrode is located at the outermost peripheral portion.
FIG. 4 is a longitudinal cross-sectional view of the structure of a conventional GCT device, including a gate driver. Since a GCT device 1P has a structure bilaterally symmetrical on a central axis CA, only one side of the structure is shown in FIG. 4.
Each reference numeral or character in FIG. 4 indicates each element as follows: 2 is a gate drive device for controlling the GCT device 1P; 3 is a stack electrode for pressurizing the GCI device 1P and drawing current; and 4 indicates a semiconductor substrate (wafer). On the peripheral portion of a first major surface of the semiconductor substrate 4, a ring-shaped gate electrode 4a of aluminum is formed in contact with a gate electrode region, and on the inner portion of the first major surface outside the gate electrode 4a, a plurality of cathode electrodes 4b are concentrically formed. The reference numerals 5 and 6 are a cathode distortion buffer plate (or cathode metal plate) and a cathode post electrode, respectively, which are sequentially stacked one above the other on the cathode electrodes 4b on the semiconductor substrate 4. Across a second major surface (opposite to the first major surface) of the semiconductor substrate 4, an anode electrode (not shown) is formed on which an anode distortion buffer plate (or anode metal plate) 7 and an anode post electrode 8 are sequentially stacked one above the other. The reference numeral 9 is a ring gate electrode which is, at its first surface (bottom surface), in surface to surface contact with the gate electrode 4a on the semiconductor substrate 4. The reference character 10P is a ring-shaped gate terminal made of a metal plate. The inner end portion of an inner plane portion 10PI of the ring-shaped gate terminal 10P is slidably located on a second surface (opposite to the first surface) of the ring gate electrode 9. Further, an elastic body 11 such as a disc or wave spring pushes the ring gate electrode 9 against the gate electrode 4a via a ring insulator 12, in conjunction with the inner end portion of the inner plane portion 10PI of the ring-shaped gate terminal 10P. This establishes an electrical connection between the gate electrode 4a, the ring gate electrode 9, and the ring-shaped gate terminal 10P. The reference numeral 13 is an insulation sheet for insulating the ring gate electrode 9 from the facing cathode distortion buffer plate 5 and the facing cathode post electrode 6. In addition to the inner plane portion 10PI, the ring-shaped gate terminal 10P includes an intermediate or fixed portion 10PF and an outer plane portion 10PO. In a portion of the inner plane portion 10PI which is not in surface to surface contact with the ring gate electrode 9, a bent portion 10Pd is formed, and in the middle of the outer plane portion 10PO, a bent portion 10Pa is formed.
The reference numeral 14 is an insulation cylinder of ceramics which is divided so as to sandwich the intermediate portion 10PF of the ring-shaped gate terminal 10P from above and below. The insulation cylinder 14 further has a protrusion 14a. The fixed portion 10PF of the ring-shaped gate terminal 10P and the insulation cylinder 14 are hermetically fixed to each other by brazing. Further, in a portion of the outer plane portion 10PO of the ring-shaped gate terminal 10P which is spaced slightly inward from the outer end portion, a plurality of mounting holes 10Pb for coupling the ring-shaped gate terminal 10P to the gate drive device 2 are equally spaced along the circumference. Further, an end portion 14b1 of a first L-shaped portion which is protruded above the upper surface of the insulation cylinder 14, bending outwardly, and one end portion of a first ring-shaped flange 15 are hermetically fixed to each other by arc welding. Likely, an end portion 14b2 of a second L-shaped portion which is protruded below the lower surface of the insulation cylinder 14 and one end portion of a second ring-shaped flange 16 are hermetically fixed to each other by arc welding. The other end portions of the first flange 15 and the second flange 16 are fixed to cut portions of the cathode post electrode 6 and the anode post electrode 8, respectively. Thus, the GCT device 1P is enclosed to be kept from the outside. The interior of the device can be replaced by inert gas.
Further, the reference numeral 17 is a plate control electrode made of a ring metal plate which is located to be concentric with the ring-shaped gate terminal 10P. The stack electrode 3 allows the plate control electrode 17 to be in pressure contact with the cathode post electrode 6. Further, like the plate control electrode 17, a plate control gate electrode 18 made of a ring metal plate is located to be concentric with the ring-shaped gate terminal 10P. An inner end portion of the plate control gate electrode 18 is electrically connected to the outer end portion of the outer plane portion 10PO of the ring-shaped gate terminal 10P. This electrical connection is established by the following members 19 and 20: 19 is an insulating sleeve for insulating the ring-shaped gate terminal 10P and the plate control gate electrode 18 from the plate control electrode 17; and 20 is a coupling part including bolts and nuts, for electrically connecting the ring-shaped gate terminal 10P and the plate control gate electrode 18 via the insulating sleeve 19 between the plate control electrode 17 and the plate control gate electrode 18. Each bolt of the coupling part 20 comes through a mounting hole 10Pb, and a mounting hole provided on the plate control gate electrode 18 corresponding to the mounting hole 10Pb. Thus, the plate control electrode 17 and the plate control gate electrode 18 are directly coupled to the gate drive device 2.
As the material for the ring-shaped gate terminal 10P, an alloy of iron and 42% nickel having a similar thermal expansion characteristic to a thermal expansion coefficient of alumina and relatively high processability and strength has been generally used to obtain high fixing strength of the brazed joint between the ring-shaped gate terminal 10P and the insulation cylinder 14 of alumina (ceramics).
While the development of the aforementioned GCT thyristor has made possible improvements in capacity and speed of a power electronics semiconductor device, larger capacity and higher speed has still been required for the GCT thyristor. This requirement causes new problems to be described below.
(1) First Problem
In a conventional contact pressure type semiconductor device like the GCT thyristor shown in FIG. 4, as previously described, the alloy of iron and 42% nickel has been used as the material for the ring-shaped gate terminal 10P. The reason for selecting this material comes from the following design consideration. That is, to ensure sufficient fixing strength of the brazed joint between the insulation cylinder 14 of alumina and the ring-shaped gate terminal 10P, the ring-shaped gate terminal 10P has to be made of a material having a similar thermal expansion coefficient to alumina (about 6.5.times.10.sup.-6 /.degree. C.). The material meeting this requirement is, for example, metal such as molybdenum and tungsten. However, not practical in terms of cost, such metal cannot be used for the ring-shaped gate terminal 10P. Then, used as an alternative is an alloy of iron and 42% nickel with the thermal expansion coefficient of 4.times.10.sup.-6 to 13.times.10.sup.-6 /.degree. C. (30.degree. C. to 800.degree. C.) which is approximate to that of alumina. In this case, it has already been verified that the strength at the brazed joint is not affected by an increase in temperature cycle. In this respect as well as in terms of cost and processability, the alloy of iron and 42% nickel has been considered as a suitable material. We can say that such consideration or selection of the material has been just a matter of design in view of cost performance.
The alloy of iron and 42% nickel is, however, a ferromagnetic material having a relatively high maximum permeability of about 40,000. The ferromagnetic property of this alloy has never been taken into consideration when designing either the conventional GTO thyristor or the GCT thyristor shown in FIG. 4. This is because an induction heating function due to electromagnetic induction at the ring-shaped gate terminal 10P has never been considered controversial in controlling low current at an operating frequency of about several hundreds Hz. However, at the operating frequency of 1 kHz or more (e.g., the maximum of 5 to 10 kHz), for example, repetitive phase inversion of gate current during a short period of time causes fluctuations in magnetic state. As a result, magnetic and electric energy stored as an iron loss in the ring-shaped gate terminal 10P is converted into thermal energy. This increases the temperature of the ring-shaped gate terminal 10P. The generation of heat in the ring-shaped gate terminal 10P is transmitted via the ring gate electrode 9 to the semiconductor substrate 4, thereby causing an increase in the temperature and the temperature difference across the surface of the semiconductor substrate 4. This brings about changes in the electrical characteristics of each segment in the semiconductor substrate 4.
Through an experiment using a copper plate, instead of a semiconductor substrate, as an element, the inventors of the present invention have found that the temperature of the ring-shaped gate terminal 10P is increased by the induction heating function. More specifically, we have found the increase in the temperature at each measuring point, by passing current of about 1 kA between the anode and the cathode of the element at the operating frequency of 1 to 10 kHz for about five minutes and measuring the temperature of an anode copper block. The data obtained by this experiment is shown in FIG. 5. In the drawing, the symbols ACu1 to ACu4, AF1 to AF4, and G1 to G4 indicate observation points (installed positions of thermocouples) shown in the plan view of FIG. 6. It can be seen from FIG. 5 that the temperature increase .DELTA.T goes almost linearly with the operating frequency (from 1 to 10 kHz). While the experimental data in FIG. 5 directly shows the likelihood that the temperature of the ring-shaped gate terminal 10P is increased by the induction heating function, we also consider that the likelihood of an increase in the temperature of the semiconductor substrate can be inferred from this data.
Then, there arises a necessity of cooling down such an increase in the temperature. The structural restrictions due to the shape and location of each element shown in FIG. 4, however, makes it difficult to directly cool down the ring-shaped gate terminal 10P.
Therefore, the selection of the material for the ring-shaped gate terminal becomes important, in terms of prevention of the induction heating function due to the electromagnetic induction, and also in terms of maintenance of the fixing strength at the brazed joint between the ring-shaped gate terminal and the insulation to the same extent as the conventional device.
(2) Second Problem
Further, the increase in the capacity of the GCT device with the increase in the maximum cut-off current brings about an increase in the number of segments to be concentrically connected in parallel with each other in the semiconductor substrate. This inevitably encourages an increase in the diameter of a package as well as of the semiconductor substrate. As the outer diameters of the semiconductor substrate and the package increase, a greater distortion which cannot be ignored remains at the brazed point between the ring-shaped gate terminal 10P and the insulation cylinder 14, and in the inner plane portion 10PI. Such a residual distortion acts as a thermal stress on the outer plane portion 10PO of the ring-shaped gate terminal 10P during operation. This very possibly causes plastic deformation of the outer plane portion 10PO.
Indeed the outer plane portion 10PO and the inner plane portion 10PI of the ring-shaped gate terminal 10P have the bent portions 10Pa and 10Pd to reduce the concentration of external stresses on the fixed portion 10PF of the ring-shaped gate terminal 10P fixed to the insulation cylinder 14. However, when the outer diameter of the ring-shaped gate terminal 10P exceeds 200 mm, for example, those bent portions 10Pa and 10Pd are insufficient to reduce the isotropic thermal stress to be imposed on a portion ranging from the inner plane portion 10PI or the fixed portion 10PF fixed to the insulation cylinder 14 to the outer plane portion 10PO. This may cause the plastic deformation of the outer plane portion 10PO, resulting in a contact failure due to the mechanical stress at the junction (feeding point) between the mounting hole 10Pb and the coupling part 20 for coupling the GCI device 1P to the gate drive device 2. Accordingly, the isotropic supply of gate current is very possibly prevented. Further, the thermal stress left at the time of brazing the ring-shaped gate terminal 10P and the insulation cylinder 14 may cause a swell in the shapes of the ring-shaped gate terminal 10P and the insulation cylinder 14, thereby making it difficult to produce an integrated part consisting of the ring-shaped gate terminal 10P and the insulation cylinder 14.
Therefore, it becomes necessary to previously prevent or suppress these problems.